Issue displaying geometry in edit mode
Thank you for your help. On reviewing the post suggested, I found the solution. 

It appears that the 3D clipping property needs to be within reasonable ranges, otherwise the edit mode display will be deformed; my viewport clipping ranged between 1 cm and 10 kilometers at the time I experienced the problem; when I dropped the end-clipping to 1 km, the problem with the weird display in the viewport disappeared. 

It is interesting to note that exactly the same happens with camera view; if the camera clipping settings are outside a certain range, the geometry will show the double-border in edit mode, although only in camera view. 

I found that The clipping range 1cm to 1km in metric (or .001 and 1000 in generic blender units) works well for both viewport and camera views.

I hope this information is of use.

Regards,

Sept